The objective of this research is to characterize and estimate the smallest radiation doses at which sparing of early and late normal tissue reactions by dose fractionation occur. Emphasis is placed on experimental design and analysis techniques for obtaining dose-response curves from in vivo assays of acute and late injury after fractionated radiation. Techniques will be developed for obtaining quantitative dose-response information and RBE(n/gamma) estimates from qualitative assays of early and late responses, to allow estimation of the limit of effective fractionation. The results of these studies may ultimately provide information to improve therapeutic results through understanding of the effects of changes in dose fractionation on normal tissues.